Many industrial processes require exact mixing and control of the delivery of various process fluids. For example, in the pharmaceutical and semiconductor industries, process fluids must be precisely mixed and the delivery of the mixed process fluid to a process tool must be precisely controlled. As used herein, a fluid can be any type of matter in any state that is capable of flow such as liquids, gases, powders, and slurries, and comprising any combination of matter or substance to which controlled flow may be of interest. Commonly, two or more components must be mixed, typically in a mixture/holding tank, to form a desired solution mixture for a particular process.
Various means for mixing such fluids are known in the art. Both intrusive and non-intrusive means have been used to mix fluids, including colloidal suspensions, to prevent separation of homogeneous solutions into constituent components and/or to reconstitute solutions that have separated into constituent elements. Intrusive mixing devices, or those objects and devices which are inserted into a fluid to agitate the fluid with the assistance of an external power source, are well known.
In the semiconductor industry, for example, it is common to mix certain process chemicals, such as slurries used for chemical mechanical polishing (“CMP”). CMP is a semiconductor processing technology in which a wafer surface is smoothed using a combination of mechanical and chemical forces, and has long been an essential process in the production of semiconductor chips.
Typically the chemicals required for such CMP processes are prepared in a batch process where a relatively large supply is prepared in mixing/holding tanks and stored for later use. Tanks holding, for example, 500 liters and 265 liters are commonly used, usually having conical-shaped bottom with a discharge port at the apex of the cone to promote complete draining. More recently, it has become increasingly common for users of these types of mixed process chemicals, to prepare a very small first batch, for example, a batch of only 25 to 50 liters. This allows a user to mitigate any impact from any variation in the process chemicals until the mixing process has stabilized. Further, bringing multiple process lines online at the same time is often beyond the capabilities of the pumping system so process lines are brought online one at a time. After the small initial batch, as the process ramps up and/or the process chemical mix stabilizes, the subsequent batches will usually gradually increase in size.
FIG. 1 shows a prior art mixing tank 10 having a multiple jet mixer 22 that is used to mix process chemicals together. Once the process materials have been adequately mixed, the process material is typically transferred from the mixing/holding tank 10 through a discharge port 14 on the base 54 of the tank via a pump 30 to a global loop 40 to the final points of use for its intended application. Points of use may be any location where there is demand for a supply of the blended process material. For example, points of use may include process machinery or tools 32 in a semiconducting fabrication facility.
As the process fluids are drawn from the tank through the discharge port, a vortex typically forms in the fluid above and along the centerline of the discharge port. A vortex is a smooth, roughly conical, rotating liquid void that forms in a fluid body as a result of a low pressure area. If the fluid level in the tank is low enough, the vortex will reach the surface and draw air (or whatever gas is in the tank) down through the fluid and out through the discharge port. Air in the process fluid delivery system is highly undesirable for a number of reasons. For example, the presence of air in the system can result in oxidation of certain chemical mixtures thereby changing the chemical reactivity and composition of the fluid, it can cause agglomeration of the slurries, and it can cause difficulties in maintaining proper fluid pressure and flow. Entrapped air can also cause the pump, used to draw out the process fluid, to lose prime and stop moving the fluid; this can reduce the effective surface area if it collects inside a filter housing. Fluids, and in particular, colloidal suspensions such as slurries used in CMP of semiconductor wafers, are most effective when delivered to CMP tools in a homogenous state, with no air in the supply line delivering fluid to these tools.
A number of methods are known in the prior art for reducing air entrapment resulting from vortex formation. One such method is described in U.S. Pat. No. 6,536,468 to Wilmer et al., for “Whirlpool Reduction Cap” (Mar. 25, 2003) (“Wilmer I”), which is assigned to the assignee of the present invention and incorporated herein by reference. Wilmer describes a cap with openings in the sidewalls that can be used to cover the discharge port and discourage vortex formation. For very low fluid levels, however, the Wilmer cap still allows the production of a vortex that can lead to air entrapment. Other types of vortex suppression devices are also known.
Accordingly, what is needed is an improved method and apparatus for preventing the formation of a vortex and resulting air entrapment in a processing fluid tank at low fluid levels.